Hi,
Desktop client caches the information about local documents w.r.t their server counterparts a.k.a index. When Desktop Client starts it uses navigates through server documents and updates\creates indexes for all the files. This is termed as indexing.
Based on number of folders\documents indexing can take more time but not 10-15 mins is too much.
How many documents do you have you in the repositories ?
Regards, Ashwani Kr Sharma